Lines due to impurities, if any are thus traced, are marked for omission from the map and their true sources recorded, while any line that is observed to vary in length and thickness in the various photographs is at once suspected to be an impurity line, and if traced to such is likewise marked for omission. I give a case.
The two lines H and K (3933 and 3968), assigned both to iron and calcium by Ångström, are proved to belong to calcium in the following way:
a. The lines are well represented in the spectrum of commercial wrought iron, but are absolutely coincident with two thick lines in the spectrum of calcium chloride with which the iron spectrum was confronted.
b. The lines are represented by mere traces in the spectrum of a specimen of pure iron prepared by the late Dr. Matthiessen. Both poles of the lamp were of iron, the lower pole consisting of an ingot of the metal which had been cast in a lime-mold.
c. The lines are altogether absent in a photograph of pure iron, where both poles of the lamp were of the pure metal not cast in lime, and they are likewise absent in a photograph of the spectrum of the Lenarto meteorite.
By eliminating lines due to impurities in the manner just described, a spectrum is at length obtained, of which every line is assignable to the particular element photographed, the same temperature being employed in the case of all the elements observed.
With regard to the second line of work, I should commence by stating that from a beautiful series of researches carried on by several methods, Mitscherlich concluded in 1864 that every compound of the first order, heated to a temperature adequate for the production of light, is not decomposed, but exhibits a spectrum peculiar to this compound.
In some experiments of my own, communicated to the Royal Society in 1873, I observed:
First. That whether the spectra of iodides, bromides, etc, be observed in the flame or a weak spark, only the longest lines of the metals are visible, showing that only a small quantity of the simple metal is present as a result of partial dissociation, and that by increasing the temperature, and consequently the amount of dissociation, the other lines of the metal appear in the order of their length with each rise of temperature.
Secondly. I convinced myself that while in air, after the first application of heat, the spectra and metallic lines are in the main the same, in hydrogen the spectra are different for each compound, and true metallic lines are represented according to the volatility of the compound, only the very longest lines being visible in the spectrum of the least volatile compound.
Thirdly. I found that with a considerable elevation of temperature the spectrum of the compound faded almost into invisibility.
